WASHINGTON  Flush with victory built on incursions in the South and West, Congressional Democratic leaders promised to use their new power to join President-electBarack Obamain pursuing an aggressive agenda that puts top priority on the economy, health care, energy and ending the Iraq war.

Al Franken, the former comedian, was in a close race for the United States Senate in Minnesota.

By reaching deep into traditionally Republican turf, the Democrats in Tuesday's elections expanded their majorities in both the House and the Senate. They picked up at least five Senate seats, in Colorado, New Hampshire, New Mexico, North Carolina and Virginia. And they picked up at least 19 House seats, with new Democrats coming from Alabama, Arizona, Colorado, Nevada, New Mexico, North Carolina and Virginia.

The full extent of the new Democratic majorities remained unknown, with tight Senate races still undecided in Alaska, Minnesota and Oregon and a runoff scheduled on Dec. 2 in Georgia. At least six House races remained too close to call.

Still, the promise of strong control of Congress also left Democratic leaders grappling with challenges of balancing a wider spectrum of views within their own party while confronting a diminished House Republican conference now decidedly more conservative.

The exuberance of Tuesday night's victories was also tempered by unease over the public's high expectations for a party in control of both Congress and the White House amid economic turmoil, two wars overseas and a yawning budget gap.

On the day after the election, leadership battles were breaking out across Capitol Hill as lawmakers contemplated the prospects of new power and opportunity. The quick start to the skirmishing signaled that some of the more bitter fights in the next Congress could be internal battles among Democrats.

For instance, Democratic aides said that RepresentativeHenry A. Waxmanof California was expected to challenge RepresentativeJohn D. Dingellof Michigan, the longest-serving House Democrat, for chairmanship of the Energy and Commerce Committee. Energy issues are expected to be a major focus of the Obama administration.

And before the week is out, Democrats could try to oust SenatorJoseph I. Liebermanof Connecticut, the independent who campaigned for SenatorJohn McCain, from the chairmanship of the Homeland Security and Governmental Affairs Committee.

SpeakerNancy Pelosi, who spoke with Mr. Obama by phone on Wednesday morning, said that they had made plans to discuss coordinated efforts for the transition and the new Congress, but that a more ambitious agenda would unfold next year.

"Our priorities have tracked the Obama campaign priorities for a very long time," Ms. Pelosi said at a news conference where she cited the economy, health care, energy and the Iraq war as topping the agenda.

She said Democrats were talking with the Bush White House about a potential $61 billion economic stimulus that could be approved in a lame-duck session.

But Ms. Pelosi said Democrats could open the 111th Congress in January with efforts to adopt measures blocked by President Bush, including ones to expand theState Children's Health Insurance Programand embryonicstem cellresearch. She said Democrats had no choice but to chart a centrist course. "The country must be governed from the middle," she said. But Democrats on both sides of the Capitol were just beginning to digest the new faces in their expanded caucuses.

Those new members include Jim Himes, a Harvard- and Oxford-educated former Goldman Sachs banker turned affordable-housing advocate who ousted RepresentativeChristopher Shaysof Connecticut, the only Republican House member in New England.

But even as Democrats tightened their grip on the traditionally liberal Northeast, roughly one-third of this year's gains in the House came in the West, including two seats in New Mexico and one each in Arizona, Colorado, Idaho and Nevada.

In Idaho, the Democrats scored an unlikely House victory when Walt Minnick, a self-described "gun-owning outdoorsman" who once worked in the administration ofRichard M. Nixondefeated Bill Sali, a Republican incumbent.

Mr. Minnick, who emphasized his résumé as a businessman and longtime executive in the lumber industry, will join a Democratic conference long dominated by urban liberals and led by Ms. Pelosi, of San Francisco.

The Senate majority leader,Harry Reidof Nevada, and other Democrats pointed to their successes in the West as evidence that they were building an enduring majority. They said new lawmakers from the region would bring a pragmatic approach driven less by partisanship and more by common sense.

Representative Tom Udall, a Democrat who won a Republican-held Senate seat in New Mexico, said, "I feel like I am coming in as a Western problem-solver, as somebody who has had success working across the aisle on many issues in my home state." Mr. Udall's cousin RepresentativeMark Udallwon the Senate race in Colorado.

Gov.Brian Schweitzerof Montana, a Democrat who won a second term on Tuesday, said the results showed that Republicans no longer had a guaranteed hold on the West. "When Democrats win in Idaho, that means that there is not a single place that's safe left anywhere," Mr. Schweitzer said.

New Mexico was a showcase of Democratic strength in this election, partly because of strong support from Hispanics, as the party won a Senate seat and two more House seats, turning the state's Congressional delegation thoroughly blue.

But even as Mr. Reid was crowing about gun-loving Democrats in the West, Senator Sherrod Brown, Democrat of Ohio, was part of a separate conference call focusing on how many Democrats won by embracing progressive economic policies.

Mr. Brown said that he expected Republicans and more conservative Democrats to join an array of legislation related to alternative energy, trade, jobs and tax policy. "With a popular president leading," he said, "we are going to see all but the most closed-minded Republicans joining us."

Representative Chris Van Hollen of Maryland, chairman of the Democratic Congressional Campaign Committee, said he believed that the new House majority would coalesce on most major economic issues but that some disagreements were inevitable.

"Clearly we are a big-tent party, and when it comes to social issues there will be some different perspectives in the caucus," Mr. Van Hollen said.

Although Democrats fell short of their goal of a 60-vote Senate majority, which would have given them the power to break filibusters, Ms. Pelosi said it would be far easier to get Republican support for Democratic bills with Mr. Bush out of office. She said Republicans often blocked bills to protect the president.

House and Senate Democrats said they believed the Obama administration and Congressional Democrats could mesh in a way that Capitol Hill Democrats and the Carter and Clinton administrations could not. As senators, Mr. Obama and Vice President-electJoseph R. Biden Jr., built strong relationships on Capitol Hill.

Republicans are already warning that Mr. Obama, a relatively junior lawmaker, will be outmaneuvered by more experienced operators on Capitol Hill, a proposition Democrats dismissed, noting that Mr. Obama would benefit from the counsel of Mr. Biden, a longtime senator from Delaware. "I think both sides realize we need one another and both sides realize that we better not blow this," said SenatorCharles E. Schumerof New York.

More Articles in US »A version of this article appeared in print on November 6, 2008, on page A1 of the New York edition.